1. Field of the Invention
This invention resides in the field of flame retardant polymers. More particularly, it relates to flame retardant polymer molding compositions which include graft copolymers of isotactic or syndiotactic polypropylene and brominated vinyl aromatics such as brominated styrenes.
2. Description of the Prior Art
By way of background, polypropylene has proven to be one of the most useful and versatile polymers. Its physical properties make it ideal for many applications including molded articles, spun fibers, hot melt adhesives and many others. These properties include, for instance, good surface appearance and solvent and stain resistance, and low moisture absorption. However, polypropylene does not possess adequate flame retardancy for certain applications. In view of its other desirable physical properties, it has naturally been a matter of great interest to provide polypropylene compositions with greater flame retardancy.
Improvement of flame retardancy has relied on modifications to polypropylene, or on additives for the polypropylene, but disadvantages have been associated with both approaches. Although a vast number of modified polypropylene compositions have been described or theorized in the prior art, few if any suitable flame retardant polypropylene derivatives have been identified. Similarly, numerous additives for increasing the flame retardancy of polypropylene have been studied and some are commercially available. Nonetheless, there is at present no commercially available flame retardant additive for polypropylene which provides adequate retention of polypropylene's physical properties, and demonstrates high thermal stability, non-migration of additive to the surface, and absence of solids at processing temperatures. The present invention contemplates a modification of polypropylene which yields a composition that retains the desirable physical properties of polypropylene, and avoids the disadvantages of alternate approaches.
In particular, the modified polypropylene of the invention avoids the frequently encountered migration or "bloom" of inert additive-type flame retardants to the surface of molded articles. Such bloom leads to unsightly surface discoloration on articles molded from the polypropylene and thereby effectively limits the amount of additive which can be used. Further, these inert additives frequently remain solid at Processing temperatures, and can damage or foul processing equipment. For example, inert additives remaining solid at processing temperatures are known to cause problems by clogging spinnerettes used in equipment for producing spun fibers. This type of equipment fouling not only reduces the efficiency of processing but can also necessitate the costly refurbishment or premature replacement of equipment.
The applicants' preferred modified polypropylenes also avoid many other problems encountered in the prior art by having only low levels of unreacted styrene monomer, typically less than 1% by weight. For example, by this aspect the applicants' invention provides a vehicle to avoid monomer juicing problems known to occur in prior art graft modified compositions. It is also significant that the compositions of the present invention can be efficiently processed without the release of excessive volatile monomer into the surrounding environment, which can be hazardous to those working with or near the materials. The prior art has failed to appreciate these substantial advantages of the compositions of the present invention.
As noted above, known flame retardant additives for polypropylene have recognized drawbacks. One such additive is hydrated alumina, which retards flame by releasing water under fire conditions. However, high loadings of hydrated alumina are necessary to give desired efficacy, and this results in poor physical properties of the polypropylene and articles molded therefrom. Among other defects, this results in undesirable changes in physical properties such as excessive stiffness, a reduction in tensile elongation, an increase in specific gravity, and a loss of "living hinge" capability.
Certain other available additives remain solid at normal polypropylene processing temperatures and thus complicate processing. Such additives include, for example, a bisimide-containing aliphatic bromine additive known as BN-451 from Ethyl Corp. of Sayreville, New Jersey, and a ring brominated polystyrene additive known as Pyro-Chek 68PB from Ferro Corp. of Cleveland, Ohio. The latter use of ring brominated polystyrene as an additive to polypropylene, rather than as a graft onto polypropylene, is a particularly clear demonstration of the failure of the prior art to recognize the present invention. Other available additives, such as decabromodiphenyl oxide, not only remain solid at processing temperatures but also are known to rise or "bloom" to the surface of molded articles. In U.S. Pat. No. 3,474,067, issued to Praetzel et al. on Oct. 21, 1969, there is described the use of ungrafted halogenated polystyrene homopolymer as a flame retardant additive for polyolefins in general, including polypropylene.
Aside from these inert additives, reports exist in the literature of attempts to chemically bond or graft flame retardants to polypropylene. To the applicants' knowledge, none of these techniques has been commercialized. For instance, M. Hartmann, et al., Z. Chem., 20(4), 146-7 (1980), report preparing graft copolymers of atactic polypropylene and four respective vinylphosphonic acid derivatives. Two of the four copolymers prepared were reported as self extinguishing when containing greater than 3% by weight phosphorous. P. Citovicky et al., Thermochim. Acta.. 93, 171-4 (1985), disclose a two-step procedure in which glycidyl methacrylate was grafted to isotactic polypropylene followed by reaction with various flame retardants including bromoacetic acid, 3,3',5,5'-tetrabromo-2,2'-dihydroxybiphenyl, dichloroacetic acid, or phenyldihydrogen phosphate. The copolymer reacted with phenyldihydrogen phosphate gave the highest limiting oxygen index value and was also reported the most thermally stable. In general, this technique is not particularly advantageous since it requires two steps and the flame retardant must be a functionalized molecule capable of reaction with an epoxide.
B. J. Hill et al., Comm. Eur. Communities [ReP.]EUR, EUR 6718 (1980), report irradiation grafting of bis(2-chloroethyl)vinylphosphonate to polyester and polypropylene fabrics to render them self-extinguishing. The authors report that the bis(2-chloroethyl)vinylphosphonate had poor reactivity toward the fabrics. Comonomers were therefore required which in some instances diminished flame retardancy and/or stiffened the fabrics.
K. Nakatsuka et al., Japan JP 44/3965 (Feb. 19, 1969), report air oxidizing polypropylene at elevated temperatures to introduce peroxy groups to the polymer followed by graft polymerization with CH.sub.2 CClCO.sub.2 Me. The product was reported to be self-extinguishing.
Outside of the field of flame retardancy, various modifications to polyolefins have been proposed. For example, U.S. Pat. No. 4,179,401, issued to Garnett et al. in 1979, relates to a process for producing a heterogenous catalyst for the hydrogenation, hydroformylation, isomerization, cracking or dehydrogenation cf organic molecules. The Garnett process comprises the steps of radiation grafting a monomer having an alpha- unsaturated bond to a metal or an organic polymer and complexing a nitrogen, halogen, or phosphorous containing group to the monomer. The Garnett et al. patent lists many possible polymer/monomer combinations. Identified polymer substrates included polyvinyl compounds, polyolefins, polyvinylidenes, polysiloxanes, polydienes, polyethers, polyimides, polysulphones, polyesters, polyamides, polyurethanes, polycarbonates and polyureas. Listed as possible monomers for use in the described process were p-nitrostyrene, p-amino styrene, p-chlorostyrene, vinyldiphenylphosphine, cis-bis (1,2-diphenylphosphino) ethylene, triallylphosphine, divinylphenylphosphine and many more.
Similarly, U.S. Pat. No. 3,177,270, issued to Jones et al. in 1965, describes a method for modifying polyethylene and other substrates for the purpose of improving tensile strength, elongation and/or flexural modulus. The Jones et al. patent specifically described the preparation of ethylene polymer modified with styrene, a styrene/acrylonitrile mixture, dichlorostyrene or a mixture of isomeric vinyltoluenes. The Jones et al. patent additionally lists other possible polymeric substrates for use in the described method as including polypropylene, polyisobutylene, polybutene, and copolymers of ethylene and propylene, ethylene and butene, ethylene and styrene, ethylene and vinyl acetate, and ethylene and methyl methacrylate. Possible graft monomers are listed as including styrene, vinyltoluene, vinylxylene, ethylvinylbenzene, isopropyl styrene, para-tert-butyl styrene, dichlorostyrene, bromostyrene, fluorostyrene, or mixtures thereof with acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, methyl methacrylate or maleic anhydride.
In U.S. Pat. No. 4,279,808, issued to Hornbaker et al. on Jul. 21, 1981, there is described a method for the preparation of polybromostyrene resin by the addition polymerization of nuclear brominated styrene. The Hornbaker et al. patent is limited to the addition polymerization of bromostyrene in the presence of specified rubbery polymers, namely SBR rubber (butadiene-styrene copolymers), EPR rubber (ethylenepropylene copolymers), EPDM rubber (i.e. terpolymers of ethylene, propylene and a diene monomer), polyisoprene rubber (e.g. cis 1,4 polyisoprene and trans-1,4-polyisoprene), Neoprene (i.e. polymers and copolymers of 2-chloro-1,3-butadiene), cis-1,4-polybutadiene, and polybutadienes having mixed structures (e.g. cis-1,4; trans-1,4 and 1,2 structures), with the polybutadienes being particularly preferred.
As is evident from the foregoing, past efforts to provide a polypropylene composition with improved flame retardancy have not been fully satisfactory. Available inert flameproofing additives have exhibited drawbacks such as bloom and interference with desired physical properties. Additionally, polypropylene materials have not been provided with grafted fire retardants which perform as well as the present inventive compositions. Accordingly, there has remained a need for fire retardant polypropylene compositions demonstrating good physical properties, and the applicants' invention addresses this need.